Is it usable already? Any known limitation?

How to install?

It's very easy. See the project homepage
for instructions and links to the precompiled packages.

Can I help?

Sure! Send patches and/or drop me a line at aar@cpan.org. You can also
find me in #reprap on FreeNode with the nickname Sound.

What's Slic3r license?

Slic3r is licensed under the GNU Affero General Public License, version 3.
The author is Alessandro Ranellucci (me).

How can I invoke slic3r.pl using the command line?

Usage: slic3r.pl [ OPTIONS ] file.stl
--help Output this usage screen and exit
--save <file> Save configuration to the specified file
--load <file> Load configuration from the specified file. It can be used
more than once to load options from multiple files.
-o, --output <file> File to output gcode to (by default, the file will be saved
into the same directory as the input file using the
--output-filename-format to generate the filename)
-j, --threads <num> Number of threads to use (1+, default: 4)
Output options:
--output-filename-format
Output file name format; all config options enclosed in brackets
will be replaced by their values, as well as [input_filename_base]
and [input_filename] (default: [input_filename_base].gcode)
--post-process Generated G-code will be processed with the supplied script;
call this more than once to process through multiple scripts.
--export-svg Export a SVG file containing slices instead of G-code.
Printer options:
--nozzle-diameter Diameter of nozzle in mm (default: 0.5)
--print-center Coordinates in mm of the point to center the print around
(default: 100,100)
--z-offset Additional height in mm to add to vertical coordinates
(+/-, default: 0)
--gcode-flavor The type of G-code to generate (reprap/teacup/makerbot/mach3/no-extrusion,
default: reprap)
--use-relative-e-distances Enable this to get relative E values
--gcode-arcs Use G2/G3 commands for native arcs (experimental, not supported
by all firmwares)
--g0 Use G0 commands for retraction (experimental, not supported by all
firmwares)
--gcode-comments Make G-code verbose by adding comments (default: no)
Filament options:
--filament-diameter Diameter in mm of your raw filament (default: 3)
--extrusion-multiplier
Change this to alter the amount of plastic extruded. There should be
very little need to change this value, which is only useful to
compensate for filament packing (default: 1)
--temperature Extrusion temperature in degree Celsius, set 0 to disable (default: 200)
--first-layer-temperature Extrusion temperature for the first layer, in degree Celsius,
set 0 to disable (default: same as --temperature)
--bed-temperature Heated bed temperature in degree Celsius, set 0 to disable (default: 200)
--first-layer-bed-temperature Heated bed temperature for the first layer, in degree Celsius,
set 0 to disable (default: same as --bed-temperature)
Speed options:
--travel-speed Speed of non-print moves in mm/s (default: 130)
--perimeter-speed Speed of print moves for perimeters in mm/s (default: 30)
--small-perimeter-speed
Speed of print moves for small perimeters in mm/s (default: 30)
--infill-speed Speed of print moves in mm/s (default: 60)
--solid-infill-speed Speed of print moves for solid surfaces in mm/s (default: 60)
--bridge-speed Speed of bridge print moves in mm/s (default: 60)
--bottom-layer-speed-ratio
Factor to increase/decrease speeds on bottom
layer by (default: 0.3)
Accuracy options:
--layer-height Layer height in mm (default: 0.4)
--first-layer-height-ratio
Multiplication factor for the height to slice and print the first
layer with (> 0, default: 1)
--infill-every-layers
Infill every N layers (default: 1)
Print options:
--perimeters Number of perimeters/horizontal skins (range: 0+, default: 3)
--solid-layers Number of solid layers to do for top/bottom surfaces
(range: 1+, default: 3)
--fill-density Infill density (range: 0-1, default: 0.4)
--fill-angle Infill angle in degrees (range: 0-90, default: 45)
--fill-pattern Pattern to use to fill non-solid layers (default: rectilinear)
--solid-fill-pattern Pattern to use to fill solid layers (default: rectilinear)
--start-gcode Load initial gcode from the supplied file. This will overwrite
the default command (home all axes [G28]).
--end-gcode Load final gcode from the supplied file. This will overwrite
the default commands (turn off temperature [M104 S0],
home X axis [G28 X], disable motors [M84]).
--layer-gcode Load layer-change G-code from the supplied file (default: nothing).
--support-material Generate support material for overhangs
Retraction options:
--retract-length Length of retraction in mm when pausing extrusion
(default: 1)
--retract-speed Speed for retraction in mm/s (default: 30)
--retract-restart-extra
Additional amount of filament in mm to push after
compensating retraction (default: 0)
--retract-before-travel
Only retract before travel moves of this length in mm (default: 2)
--retract-lift Lift Z by the given distance in mm when retracting (default: 0)
Cooling options:
--cooling Enable fan and cooling control
--min-fan-speed Minimum fan speed (default: 35%)
--max-fan-speed Maximum fan speed (default: 100%)
--bridge-fan-speed Fan speed to use when bridging (default: 100%)
--fan-below-layer-time Enable fan if layer print time is below this approximate number
of seconds (default: 60)
--slowdown-below-layer-time Slow down if layer print time is below this approximate number
of seconds (default: 15)
--min-print-speed Minimum print speed speed (mm/s, default: 10)
--disable-fan-first-layers Disable fan for the first N layers (default: 1)
--fan-always-on Keep fan always on at min fan speed, even for layers that don't need
cooling
Skirt options:
--skirts Number of skirts to draw (0+, default: 1)
--skirt-distance Distance in mm between innermost skirt and object
(default: 6)
--skirt-height Height of skirts to draw (expressed in layers, 0+, default: 1)
Transform options:
--scale Factor for scaling input object (default: 1)
--rotate Rotation angle in degrees (0-360, default: 0)
--duplicate Number of items with auto-arrange (1+, default: 1)
--bed-size Bed size, only used for auto-arrange (mm, default: 200,200)
--duplicate-grid Number of items with grid arrangement (default: 1,1)
--duplicate-distance Distance in mm between copies (default: 6)
Miscellaneous options:
--notes Notes to be added as comments to the output file
Flow options (advanced):
--extrusion-width-ratio
Calculate the extrusion width as the layer height multiplied by
this value (> 0, default: calculated automatically)
--bridge-flow-ratio Multiplier for extrusion when bridging (> 0, default: 1)

If you want to change a preset file, just do

slic3r.pl --load config.ini --layer-height 0.25 --save config.ini

If you want to slice a file overriding an option contained in your preset file:

slic3r.pl --load config.ini --layer-height 0.25 file.stl

How can I integrate Slic3r with Pronterface?

Put this into slicecommand:

slic3r.pl $s --load config.ini --output $o

And this into sliceoptscommand:

slic3r.pl --load config.ini --ignore-nonexistent-config

Replace slic3r.pl with the full path to the slic3r executable and config.ini
with the full path of your config file (put it in your home directory or where
you like).
On Mac, the executable has a path like this:

/Applications/Slic3r.app/Contents/MacOS/slic3r

How can I specify a custom filename format for output G-code files?

You can specify a filename format by using any of the config options.
Just enclose them in square brackets, and Slic3r will replace them upon
exporting.
The additional [input_filename] and [input_filename_base] options will
be replaced by the input file name (in the second case, the .stl extension
is stripped).

The default format is [input_filename_base].gcode, meaning that if you slice
a foo.stl file, the output will be saved to foo.gcode.